9. Ground Zero Is the Developmental State of Minimum Biological Age

Ground zero denotes the point (or period) at which the minimum biological age is achieved during embryogenesis, representing the state of minimal damage and marking the beginning of organismal aging (14–16). Conceptually, it is the lowest reachable state of the deleteriome under normal development, created by globally coordinated programs that clear up damage, rebuild the epigenome, select for genotypes, and standardize cellular states. It may be said that the preground zero phase prepares the system for organismal life. In mammals, ground zero corresponds to the developmental stage around gastrulation, where cells differentiate into germ layers, the organismal plan is established, and the germline is formed. Organismal identity (e.g., immune and later the self) and perhaps organismal life itself begin at this point. The embryonic rejuvenation phase leading to ground zero suggests possible future interventions: extending or deepening this rejuvenation phase, or engineering ground-zero-like resets in adult tissues.